NMR spectroscopy has been used for many years in the identification of compounds by comparing the spectra of known compounds with those of the compounds to be analyzed. The techniques employed in this method of spectral analysis are described in the literature, and NMR spectrometers are commercially available. For a discussion of the theory of nuclear magnetic resonance and a decription of the basic components of an NMR spectrometer and its operation, reference may be made to Van Nostrand's Scientific Encyclopedia, 4th Ed., D. Van Nostrand Company, Inc., Princeton, New Jersey. Briefly, in the operaton of a spectrometer, a tube containing a sample to be analyzed is positioned between the pole faces of a direct current electromagnet whose gap can be varied. An oscillating radio frequency field is imposed at right angles to the magnetic field. A separate radio frequency coil in the form of a few turns of wire wound tightly around the sample tube serves as the receiver coil to pick up the resonant signal from the sample. When nuclear transitions are induced, energy is absorbed from the receiver coil, causing the voltage across the receiver coil to drop. After this voltage change is amplified and detected, the resulting direct current voltage is placed on an oscilloscope. The NMR spectrum, a pattern of intensity as a function of frequency, is thereby produced. An interpretation of the spectrum makes it possible to determine the nuclei present in molecules and their relations to the remainder of the molecule.
Since the beginning of NMR spectroscopy in the late 1940's, the effects of paramagnetism on nuclear magnetic resonances have been the subject of considerable study. The object of the study has been to provide means to simplify and clarify the NMR spectrum, thereby rendering compound identification more certain as well as increasing the scope of the applicability of NMR spectroscopy. The results of the study have been the development of so-called shift reagents which, when added to a sample of a compound subjected to NMR, will cause frequency shifts that desirably will result in a high resolution spectrum without objectionable peak broadening. Although large frequency shifts caused by several paramagnetic chelates have been observed, up to the present time much of the work has revolved around the question of which metal will permit the observation of such high resolution spectra of its complexes. It has been reported [J. Amer. Chem. Soc., 91, 5160 (1969)] that the dipyridine adduct of tris(2,2,6,6-tetramethyl-3,5-heptanedionato) europium(III) produces relatively large concentration-dependent paramagnetic shifts in cholesterol monohydrate without serious peak broadening. Subsequently, it was reported [(Chem. Commun., 422 (1970)] that the coordinating effectiveness of the europium was significantly improved by elimination of the pyridine using the unsolvated europium chelate of 2,2,6,6-tetramethyl-3,5-heptanedione[Eu(thd).sub.3 ].
While the above-mentioned chelates are useful as shift reagents for specific classes of compounds, their effectiveness is drastically reduced when used with weak Lewis bases. Moreover, the solubility of the thd chelates is relatively low in nonalcoholic solutions. As a result free ligand and complexed ligand are present, a condition that limits the spectral shifts obtainable.
It is an object of this invention, therefore, to provide superior paramagnetic shift reagents for nuclear magnetic resonance spectral clarification.
Another object of the invention is to provide shift reagents that can be effectively used with organic compounds having a donor group, such as weak Lewis bases.
A further object of the invention is to provide shift reagents that are highly soluble in nonalcoholic solutions.
Still another object of the invention is to provide an improved method of spectral analysis of an organic compound having a donor group by nuclear magnetic resonance.
A still further object of the invention is to provide a composition which, when added to a compound, greatly simplifies and clarifies its spectrum.
Yet another object of the invention is to provide a composition which, when subjected to NMR, has a spectrum that can be readily interpreted.